(481.4774) Laka Foundation -The HTR is based on the technology of a gas-cooled graphite-moderated reactor. In the UK, gas-graphite reactors are the main type of reactors used in the production of nuclear electricity; only one water reactor (Sizewell B) is being used. Although the principles of an HTR are based on the gas-graphite reactor, no HTR was developed in the UK. At the German research center Juelich, an HTR, called the 'Arbeidsgemeinschaft Versuchsreactor AVR', was opened in 1967. In this reactor the fuel was embedded in graphite balls. It was a thorium reactor; neutrons from a chain reaction were used to breed fissionable uranium-233 from the non-fissionable thorium-232. Later models dropped the thorium cyclus as there were too many problems with the reprocessing of thorium fuel. Other problems with the AVR were leakages in the helium cooling circuit, a fire with turbine-oil and the production of fuel. In 1985 the Hamm-Uentrop Thorium High Temperature Reactor (THTR) was opened. This DM6 billion (US$ 3.5 billion) reactor faced serious safety problems. In May 1986 (a week after the Chernobyl accident), radioactive gas escaped from the cooling system, after graphite-fuel balls stuck in the fuel inlet. Other problems occured when fuel balls were damaged and with sticking control rods. In 1989 the reactor was permanently closed due to economic and political reasons. The Siemens company studied the HTR-Module, a 200 MW HTR, for the production of process-heat (steam) and electricity. In 1987 Siemens asked for a non-site specific license for building a prototype in the German state of Lower-Saxony. As no possible buyer was found, the license was ultimately denied. After spending some DM2 billion (US$ 1.2 billion) on the HTR, Siemens ended the project in 1991 by selling the technology to China.
In the US an HTR, developed by General Atomics, was opened in 1967 in Peach Bottom. It closed in 1974, the same year when the Fort St. Vrain HTR was opened. This reactor was closed in 1989 due to problems with the cooling system and control rods. General Atomics is still working on the development of a new HTR, but faces financial problems. In 1995 the US House of Representatives stopped subsidizing General Atomics research. Now the company hopes to sell an HTR to Russia for burning weapons plutonium. Smaller HTR projects are being developed elsewhere: the South African utility Eskom is studying the possibilities of HTR reactors, Japan has almost finished building its first HTR research reactor, and China is building a research HTR with the Siemens technology.
Technology
The HTR is not only made for the production of electricity. Due to the high temperature of the
coolant, the HTR can also be used for the production of heat. This process-heat, f.i. through the
production of steam, could be used in chemical industry, paper-mills, city heating and desalination
plants. The uranium or thorium fuel is embedded in millimeter-small fuel particles. Some thousands
of these particles are put in a graphite ball, approximately 5 centimeters in diameter. The
function of the graphite is to moderate the neutrons produced by fission. Without moderation, the
fission of uranium or thorium is impossible. The graphite balls are in the reactor vessel and are
cooled by helium gas. Cooling by water is impossible as graphite reacts heavily with water. The
heated helium is used to drive an electricity producing turbine. After this the heat is used for
the production of steam in a steam generator. As the HTR is used both for the production of heat
and power it is sometimes called a Cogeneration (or Combined) Heat and Power (CHP) plant. Some
concepts are based on a uranium-cycle, using enriched uranium. Other concepts are based on the
thorium cycle, using the neutrons from uranium fission to breed uranium-233 from thorium-232.
Although there is little experience with thorium technology, a longer future is foreseen, mainly in
India (see also WISE NC 461.4577: India; experimental thorium
reactor gone critical.), as uranium resources are getting smaller.
Safety
The HTR is often presented as an inherently safe reactor. The term inherently safe suggests that
absolutely no accidents can happen. This is of course not true; one can never exclude an accident.
The IAEA recommends not to use this term, they prefer the use of 'next generation reactors'. In the
most common reactors, water reactors, the danger of a large release of radioactivity exists if the
fuel elements melt. This can happen when the water cooling fails, for instance, due to a leak in
the coolant circuit. Due to residual heat, the fuel- elements would melt and release the fission
products. In the HTR the fuel particles are enclosed in graphite balls that cannot melt. But
graphite can burn, a property that caused the serious 1957 accident at the UK Windscale plutonium
producing reactor. The burning of graphite in the 1986 Chernobyl disaster extended the release of
radioactivity and made it difficult to fight the fire, as graphite also reacts heavily with water.
In the HTR a fire could occur when air comes into the reactor, for instance through an external
explosion or an accident with an airplane. Water can enter an HTR when leaks occur in the steam
generator. Research is being done to give the graphite balls a corrosion resistent layer. Graphite
is damaged when temperatures reach 1600 degrees Celcius. To keep the temperature under this
critical 1600 degrees, the release of residual heat from the reactor must be high in case of a loss
of coolant accident. Therefore the HTR lacks the safety containment that is used in a light water
reactor building. A safety containment would have an isolating effect on the reactor. But the
function of a safety containment is to keep radioactivity inside the reactor building in the event
of an accident, as well as give protection from forces from outside. In 1988 the US safety
authority Nuclear Regulatory Commission (NRC) doubted the safety characteristics of the HTR: the
improvement in safety by the use of graphity would be followed by a decrease in safety due to the
absence of a containment.
Proliferation
In a fission reactor uranium-238 is formed into plutonium-239 when it captures a neutron.
Plutonium-239 can be used in nuclear weapons. But other plutonium isotopes are also bred which are
unsuitable in nuclear weapons. It is the amounts of plutonium-239 and other plutonium isotopes that
make the plutonium more or less suitable for nuclear weapons. The plutonium produced in
gas-graphite reactors is especially of a high weapons quality. Therefore the HTR can be misused for
the production of weapons material. Some HTR concepts make use of higher enriched uranium, that can
also be used in nuclear weapons. In the thorium cyclus based HTR, uranium-233 is produced. This
uranium-233 is of weapons quality, just like plutonium-239. The choice for a thorium cyclus means
also a choice for reprocessing. The uranium-233 must be extracted from spent fuel for the
production of new uranium-233 fuel. This means, in addition to the environmental risks of
reprocessing, the production of pure uranium-233 with the risks of misuse and theft. The
possibility to load and unload an HTR during electricity production makes the control or misuse
more difficult than with a water reactor, which must be shut down for fuel to be unloaded.
Economics
The HTR is a reactor with which there has been very little experience worldwide. Therefore a very
big financial investment is required before production of this reactor can be economical.
Investments are not only needed for the reactor but also for a fuel production and reprocessing
infra-structure. Nowadays an HTR cannot compete with a gas-fueled Cogeneration plant. Altough the
fuel costs would be lower than in a gas plant, the main costs will come with building the reactor.
According to a Dutch study gas prices would have to rise to three times their current level before
an HTR (in serial production) could become competitive.
Conclusion
The choice of the nuclear industry to develop an HTR looks to be an attempt to show the public a
'safe' reactor that cannot melt. But complete safety can never be assured because of the
possibility of graphite burning, the lack of a containment, etc. With the HTR, proliferation risks
will increase. The environmental pollution from reprocessing will continue in the thorium concept.
And off course the uranium or thorium mining will destroy mining areas, contaminating environment
and people with radioactivity. The argument to fight the greenhouse effect with a safe non carbon
dioxide producing reactor is false. The HTR will produce radioactive waste, in volume even more
than a water reactor due to the radioactive graphite balls. This will enlarge the amount of waste
that has to be stored for millions of years. Like the greenhouse effect, another worldwide
environmental problem will be created.
Sources:
- Greenpeace press release, 11 July 1996
- Laka foundation fact-sheets; 4 July 1996 and 16 January 1997
- WISE News Communique 455.4509: Dutch dreams: A nuclear reactor in every town. ,12 July 1996
- Energy Research Foundation ECN (NL), Incogen pre-feasability study - Nuclear Cogeneration based on HTR Technology, September 1997.
Tel: +31-20-6168294; Fax: +31-20-6892179.
E-mail: laka@antenna.nl
OECD/IAEA meeting on HTR. From 10 to 14 two meetings on the HTR took place at the Energy Research Foundation (ECN) in The Netherlands. The IAEA Technical Committee Meeting dealt with the future prospects of the HTR. A second meeting (of the OECD Nuclear Energy Agency) discussed the technical aspects of research on the HTR. According to the discussions at the IAEA meeting a market share of the HTR is not foreseen in the next decade: the HTR is still in a developing stage and can not compete with other energy sources. Only one small chance for an economically realizable HTR in the next decade, is foreseen in South Africa, were it would compete with coal plants. The Dutch Energy Research Foundation presented at the meetings its research report on the 'Incogen', INherently safe Nuclear COGENeration. Whether the research on the HTR at the institute would continue was doubted when dutch government ended in 1996 a finance program for new generation reactors. Now the ECN is looking for cooperation with other European institutes and companies. Under coordination of Framatome and with finances from the European Union new research has started.
Laka Foundation, 19 November 1997
See also Reactor grade PU also suitable for N-weapons [NC 483-4.4799] for important information on this subject.
 |
 |
 |
 |
 |
|
 |
 |
 |